1. Industrial Field of Utilization
The present invention relates to a color electrophotographic apparatus applicable in color printer, color copier, color facsimile apparatus or the like, and an image forming unit for use therein.
2. Related Art of the Invention
Generally, to form a color image by electrophotography, color toner images of yellow, magenta, cyan and black are overlaid on a transfer member, and a color image is composed. One of such color image forming methods is the transfer drum method, in which color toner images are sequentially formed on one photosensitive member, and a transfer member wound on the transfer drum is repeatedly set opposite to the photosensitive member, and the color toner images formed thereon are overlaid and transferred. Other known method is the continuous overlay method, in which plural image forming parts are set sequentially, and the transfer member conveyed by a belt passes the transfer position of the image forming parts to overlay the color toner images.
An example of employing the former transfer drum method is a color image forming apparatus disclosed in the Japanese Laid-open Patent No. 1-252982. FIG. 1 shows a general structural outline of this prior art. Its constitution and operation are briefly described below. In FIG. 1, reference numeral 1 denotes a photosensitive member, 2 is a charger, 3 is a developing device, 4 is a transfer drum, and 5 is a cleaner. The developing device 3 consists of a Y developing unit 6 for making a yellow color toner image, an M developing unit 7 for magenta color, a C developing unit 8 for cyan color, and Bk developing unit 9 for black color. The entire body of the developing device 3 rotates, and each developing unit sequentially confronts the photosensitive member 1 to be ready to develop. Each developing unit has a toner hopper 10 to be refilled with toner from outside, and a color toner corresponding to each developing unit is supplied from each hopper. The toner concentration is detected by optical detecting means, on a developing roller at the developing position, and the toner is supplied from a position moved 90 degrees from the developing position. In monochromatic continuous mode, when the toner concentration is lowered, the entire developing device rotates, and the toner is supplied at the toner supply position. The toner fed from the hopper 10 is conveyed in a pipe (not shown). The pipe is supplied from the toner refill port (not shown) provided in part of the central shaft through the rotation central shaft of the rotary developing apparatus into the developing unit by gravity. The toner supplied in each developing unit is circulated in the axial direction by a screw (not shown) provided in each developing unit.
The transfer drum 4 and photosensitive member 1 rotate at a specific speed in the arrow direction while confronting each other. When the operation starts, the photosensitive member 1 rotates in the arrow direction, and its surface is uniformly charged by the charger 2. Afterwards, the surface of the photosensitive member is irradiated with a laser beam 11 modulated by signal in order to form a yellow image of the first color, and a latent image is formed. This latent image is first developed by the yellow developing unit 6 confronting the photosensitive member, and a yellow toner image is formed. Until the formed yellow toner image comes to the position confronting the transfer drum 4, already a sheet of paper is wound on the outer circumference of the transfer drum 4, as the transfer member sent from a paper feed unit 12, with its front end clipped by a claw 13. The timing is adjusted so that the yellow toner image on the photosensitive member may confront the specified position of the paper, and then the toner image is formed.
The yellow toner image on the photosensitive member is transferred on the paper by the action of a transfer charger 14. Later, the surface of the photosensitive member is cleaned by the cleaner 5, making it ready to form an image of next color. In succession, toner images of magenta, cyan and black are similarly formed, and at this time the developing device 3 sets each developing unit to confront the photosensitive member depending on the color so as to be ready to be developed. The diameter of the transfer drum is enough to take up the longest paper and exchange the developing units between formations of color images.
Irradiation of laser beam 11 for forming color images is adjusted in timing so that the toner image of each color on the photosensitive member may be matched in position with the toner image already transferred on the paper on the transfer drum. In this way, toner images of four colors are overlaid and transferred on the paper on the transfer drum 4, and a color image is formed on the paper. When toner images of all colors are transferred, the paper is peeled off the transfer drum 4 by a parting claw 15, and through the conveying unit 16, the above toner image is fixed by a fixing device 17, and the paper is discharged out of the apparatus.
The initial operation of this apparatus is described. When the apparatus power source is turned on, the fixing device 17 begins to heat up. Coming closer to the specified temperature, the apparatus starts to operate same as in the image forming operation above. At this time, the laser beam 11 is not emitted. Each developing unit sequentially opposes the photosensitive member, and an image is formed for a specific time. This action is done by each developing unit, and the apparatus stands by in a state ready for starting image formation, thereby terminating the initial action.
On the other hand, as an example of color image forming apparatus using continuous transfer system, the Japanese Laid-open Patent No. 1-250970 is disclosed. In this example, four image forming stations including the photosensitive member and light scanning means are arranged, and the paper conveyed by belt passes through each transfer unit, and toner images are overlaid.
A further different method is disclosed in the Japanese Laid-open Patent No. 2-183276. This is a method of overlaying toner images of colors sequentially formed on the photosensitive member once on an intermediate transfer unit, and finally moving them to the transfer member in batch.
Recently, aiming at downsizing and easier maintenance of such apparatus, the Japanese Laid-open Patent No. 62-287264 is disclosed. This apparatus is explained by reference to FIG. 2. Herein, plural image forming units are hold on a rotary support, and the image forming unit is exchanged for one exposure position, and a color image is formed on the paper wound on the transfer unit. In FIG. 2, reference numeral 18 is a rotary frame for rotatably supporting four image forming units 19a, 19b, 19c, 19d for cyan, magenta, yellow and black, and it is held by a support shaft 20 and rotates in coincidence with the timing of development. Units containing four color toners are identical in constitution, and only the toners are different. The construction of this image forming unit, and the image forming process are described in detail below by reference to the image forming unit 19a stopped at the image forming position.
The image forming unit 19a comprises a photosensitive drum 21, a charger 22, and a cleaner unit 23. Outside the image forming unit 19a is provided a semiconductor laser (not shown), and the signal light emitted from the laser is exposed and scanned by a polygon 24 on the photosensitive drum 21 in the axial direction. In FIG. 2, reference numeral 25 denotes a lens system, and 26 is a mirror for varying the direction of the laser beam. The signal light issued from the semiconductor laser depending on the image signal of each color exposes the photosensitive drum 21 which is charged by the charger 22, and forms an electrostatic latent image. Afterwards, this electrostatic latent image is developed by a developing device 27, and a toner image is formed on the photosensitive drum 21. On the other hand, the transfer member is wound on the surface of a transfer drum 28. The toner image formed on the photosensitive drum 21 is transferred by a transfer charger 29 provided inside the transfer drum 28. After transfer, the toner left over on the photosensitive drum 21 is cleaned by the cleaner unit 23. After being discharged an erase lamp 30, the photosensitive drum 21 stops its rotation. After such image forming action, the rotary frame 18 rotates 90 degrees, and exchanges the units. At the lowest image forming position, a unit 19b moves instead of the unit 19a, and is fixed. Next, an image is formed by using the unit 19b. According to the image signal corresponding to the color of the unit 19b, semiconductor laser is modulated, and a latent image is formed and developed, and overlaid and transferred on the transfer member wound on the transfer drum 28. The subsequent process is same as in the case of the unit 19a. In this way, the image forming process is conducted in four units, and toner images of four colors are overlaid on the transfer member, so that a color image is formed. Afterwards, the transfer member is separated from the transfer drum 28, fixed by the fixing device, and is discharged out of the machine.
This ends the brief description of the constitution and operation of the prior art.
However there is the following problems in the prior art. In the transfer drum type in FIG. 1, paper must be wound on the transfer drum, and the diameter of the transfer drum must be more than a specific size, and its structure is complicated, and hence the size was large. Yet, postcard, thick paper or tenacious paper that cannot be wound on the transfer drum could not be used. Furthermore, when exchanging the photosensitive member, it required complicated maintenance such as adjustment of each developing unit according to the characteristic of the photosensitive member. Accordingly, the maintenance required skilled specialists. Besides, the photosensitive member, developing device, and toner hopper were separately constituted. It hence required a large and complicated mechanism for sending the toner from the hopper to the developing device. Or when the apparatus was left over in the environments of high temperature and high humidity for a long time, the fluidity of the toner in the hopper was lowered, and the toner was solidified in the toner hopper and in the refill route. It caused toner supply shortage, drop of toner concentration in the developing device, and deterioration of image quality. At this time, if the toner was left over enough in the hopper, the toner remainder detecting sensor sometimes issued a wrong toner empty signal. Moreover, because of the constitution for supplying the toner into the rotating and moving developing device from a fixed external unit, the toner spilled while the developing device was moving.
Once the toner was solidified, still more, if the initial action is started by turning on the power source, the toner could not be agitated sufficiently, and the toner electric charge amount was insufficient, and a background was overlaid on the initial image. If the initial action time of each developing unit is prolonged in order to solve the problem of toner solidification, it took too much time, and induced side effects such as fatigue of photosensitive member to shorten the life of photosensitive member. Meanwhile, in the constitution in FIG. 1, if the toner in the developing device was stirred by making use of the rotary motion of the developing unit group, toner solidification could not be eliminated. Still worse, toner scattered violently from the developing roller, and it caused a problem of toner contamination inside the machine.
On the other hand, the continuous transfer method does not require the transfer drum as in FIG. 1, but it requires, instead, latent image forming means such as laser optical system for forming latent image on the photosensitive member by the same number as the number of colors, and the structure is complicated and expensive. Besides, there are plural image forming positions, and the relative positional deviation of image forming part of each color directly leads to color deviation. In particular, as shown in the example disclosed in the Japanese Laid-open Patent No. 1-250970, it requires accurate positioning of colors of latent images by the latent image forming means, and hence requires due consideration and complicated constitution of the image exposure system as latent image forming means. Along with the complicatedness of such positioning, when replacing parts such as photosensitive members at the time of maintenance, it was difficult to reproduce the accurate positioning, and complicated maintenance was needed.
Furthermore, in the example of using the intermediate transfer member disclosed in the Japanese Laid-open Patent No. 2-183276, since developing units of all colors are disposed around one photosensitive member, the shape of the photosensitive member is large. The maintenance of developing device and photosensitive member was also difficult. For example, for maintenance of the developing device, it required image adjustment and position adjustment with the photosensitive member.
In the apparatus shown in FIG. 2, the gap is wide between two adjacent image forming units for composing the optical path being held on a rotary support. On the side of the image forming unit group in the state being installed in the apparatus, various gaps are opened. Accordingly, when forming an image, the lights from the heat lamp of the fixing device, erase lamp, and others from outside expose the photosensitive member as strayed lights, which caused background on the image. In this apparatus, at the time of rotary motion for exchanging the image forming units, the toner image on the transfer member contacted with the image forming unit, and the toner image was disturbed, and a favorable color image could not be obtained.